The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for measurement of inhomogeneous magnetization transfer (“IHMT”) using MR techniques. These methods show high specificity for certain tissues and samples, such as myelin.
MRI can be used to measure the exchange of magnetization between molecules, and can thus provide unique information about the chemical and molecular environment of samples or tissues. One type of such exchange measurement is broadly referred to in the field as magnetization transfer (“MT”). This technique is capable of measuring the exchange of magnetization from spin species, such as the nuclei of hydrogen atoms (often referred to by the synonym protons), whose motion is restricted because they are bound to macromolecules. These so-called “bound pool” spin species have very short transverse relaxation times (“T2”) as a result of being tightly bound to their environment. For example, T2 can be as short as 10-50 μs for some bound pool spin species. As a result, these bound pool spins are difficult to image with traditional methods. The magnetization of bound pool spins, however, can exchange rapidly with so-called “free pool” spins that are not tightly bound to their environment. Such “free pools” exist (or are present) as, for example, water protons in many soft tissue environments.
The properties of the bound pool can thus be probed by applying RF power at a frequency offset from water. This applied power has little direct effect on the free pool, but will tend to directly saturate the bound pool. The saturated bound pool magnetization then exchanges with the free pool to thereby decrease the measurable magnetic resonance signal.
Despite extensive studies, magnetization transfer has not yielded widely recognized diagnostic benefits. As an example, for approximately a decade, there was significant interest in the use of MT for different clinical applications. One such application was the characterization of myelination as it relates to multiple sclerosis. It gradually became appreciated, however, that MT offered little specificity to imaging myelin, and that MT contrast generally was typically closely related to T1 and T2 contrasts. Although there are some signs of special sensitivity, they have generally been overwhelmed by the low contrast between myelinated and unmyelinated tissues and by the sensitivity of the MT technique to scanning imperfections. As an example of the low MT contrast between myelin and other tissue types, traditional MT shows as much or more signal in muscle and skin than in myelinated tissues.
It would therefore be desirable to provide a system and method for magnetization transfer MRI techniques that are more diverse and/or can elicit additional contrast mechanisms.